1. Field of the Invention
The present invention relates to a driving circuit and a semiconductor device with the driving circuit. The present invention more specifically relates to a technique of applying a negative bias to a power semiconductor switching element.
2. Description of the Background Art
A power semiconductor module, more specifically what is called an IPM (intelligent power module) is used for power control or motor control of industrial machines, electric railroads, automobiles, office automation, household electric appliances, and others. This power semiconductor module includes a plurality of semiconductor switching elements such as IGBTs, and a control circuit for driving and controlling the semiconductor switching elements that are housed in one package.
An enhancement element with a positive threshold is generally used as a power semiconductor switching element (IGBT or MOSFET). So, in principle, application of a negative bias to a control terminal is not required to turn the switching element off. The switching element can be turned off only by making a gate voltage not exceed a threshold (that is generally 0 V).
Meanwhile, this bias of 0 V may cause a problem during a transient phenomenon (dynamic operation). To be specific, a voltage across main terminals (collector-to-emitter voltage) increases rapidly in response to a switching operation for turn-off. At this time, electric charges are accumulated in a gate through a feedback capacitor placed between the collector and the gate to increase a gate voltage. So, the switching element in off state may accidentally be turned on.
A threshold has a negative temperature coefficient. So, a threshold decreases during a high-temperature operation, thereby making the aforementioned problem more serious.
In order to avoid this problem, a negative bias may be applied between the gate and the emitter when the switching element is turned off, thereby preventing the gate voltage from exceeding the threshold.
Meanwhile, a negative power supply is required for application of a negative bias, meaning that a driving circuit requires two power supplies including a positive power supply for applying a positive bias for turn-on and the negative power supply. This disadvantageously results in upsizing of an entire system.
The aforementioned problem may be solved by the following technique that does not require application of a negative bias. According to this technique, a control IC for supplying a gate voltage and a bare chip IGBT are arranged close to each other in an IPM. An interconnect line is placed such that the control IC and the IGBT are separated by the shortest distance, thereby reducing interconnect impedance to the lowest possible level. Thus, a short-circuit can be formed between the gate and the emitter of the IGBT at low impedance. As a result, a gate voltage is not increased even if an applied bias is 0V. Further, a single power supply can be used in a driving circuit, thereby allowing size reduction of an entire system.
Meanwhile, as a result of recent advances in semiconductor technology, semiconductor switching elements have been developed and put into practical use that operate at high speeds, and in an environment of higher temperature and higher voltage than those of a conventionally employed environment. A semiconductor switching element should be operated reliably in such an environment. The aforementioned technique of close arrangement to reduce impedance finds difficulty in maintaining a reliable operation (especially, turn-off) in a high-temperature and high-voltage environment.
This may be avoided by a conventionally employed technique that causes a control circuit to generate a negative voltage from a single power supply, and realizes driving with a negative bias by using the negative voltage thereby generated. As disclosed for example in patent literature 1 (Japanese Utility Model Application Laid-Open No. 5-48592 (1993)) or in non-patent literature 1 (data sheet EXB840, Fuji Electric Co., Ltd.), a negative voltage is generated by using a Zener voltage, and driving with a negative bias is realized by using the negative voltage thereby generated.
The circuit shown in FIG. 1 of patent literature 1 requires large-capacity by-pass capacitors (15 to 20) to stabilize a divided potential. However, lifetime may be shortened depending on the condition of use if aluminum electrolytic capacitors are used as the bypass capacitors.
The value of a resistor 33 should be reduced in order to shorten a charging time during a high-speed operation. However, reducing the value of the resistor 33 in turn increases a stand-by current that starts to flow concurrently with the turn-on of a power supply 1 supplied from a power transformer 1. This makes it difficult to reduce power consumption of the single power supply to be supplied.
Non-patent literature 1 suffers from the same problem. Non-patent literature 1 also suffers from the problem as follows. Provision of a capacitor is required in order to form a charge and discharge circuit of electric charges for the gate voltage of a switching element. In this case, a required capability of driving with a negative bias is substantially the same as that of the aforementioned bypass capacitors. This means a large-capacity capacitor should be provided, resulting in a fear of lifetime reduction.